It is known to use mass-spectroscopy for the identification of biological material such as micro-organisms (e.g. bacteria, virusses), cells, peptides etc.
MALDI-MS (Matrix Assisted Laser Desorption-Ionization-Mass Spectroscopy), for example, involves laser ionization of sample material containing micro-organisms and mass spectroscopic analysis of the ionized particles. In MALDI-MS the micro-organisms are prepared for ionization by providing a mixture of micro-organisms and matrix material on a target plate. While the mixture dries, the matrix material attaches to the micro-organisms, which enables soft ionization with relatively low energy that preserves large molecular fragments for the analysis constituents of the micro-organisms. Laser shots are fired at selected target locations on the target plate in order to ionize material from these target locations. For each target location a mass spectroscopic analysis of the resulting is performed.
MALDI-MS has been found to be very suitable for laboratory analysis, but several problems remain. For samples with an inhomogeneous mix of micro-organisms there is a serious risk that a single laser shot ionizes material from a mix of micro-organisms of different species, which complicates the identification of individual species. Even if an isolated single micro-organisms could be found on the target plate, it would not be possible to detect it in the noise of all material ionized by a laser shot. In MALDI-MS these problems are overcome by using an initial breeding step when the original sample, taken for example from patient in vivo, contains an inhomogeneous mix of micro-organisms. Breeding may be used to ensure that a substantial number of micro-organisms of mainly one strain are produced within a single laser shot. However, breeding may take over a day. As a result, practical MALDI-MS is not suitable if a short turn around time is needed between taking an in vivo sample and providing an analysis result. This limits its applicability.
For example, in an example before transferring an incoming patient to an environment where the MRSA bacteria could do harm, it would be desirable to obtain a sample from the patient, and identify MRSA bacteria among the many micro-organisms present in the sample. To be of practical use, a result of such a measurement should preferably be available within a few hours at most. The need to breed micro-organisms prevents practical use of conventional MALDI-MS for this purpose.
The use MALDI-MS for identification of micro-organisms is known from an article titled “Matrix-assisted laser deporption/ionization aerosol time of flicght mass spectroscopy for the analysis of bioaerosols: development of a fast detector for airborne biological pathogens” by A. L. van Wuijckhuijse et al and published in the Journal of Aerosol Science Vol 36 pages 677-697 (EPO reference XP004936536). This document describes the use of a nebulizer to nebulize solutions that were produced by diluting mono-culture bacterial samples to predetermined values. The nebulization mentioned in this document does not guarantee drops of a uniform size. As a result the number of bacteria per drop may vary, but this did not matter as only one species of bacteria was used. Moreover, the nebulization method of this document requires that a large sample volume is available, even if distributed over small drops.
From WO02052246 an apparatus for detecting and identifying bioaerosol particles in the air is known. This apparatus collects air containing bioaerosol particles, and feeds the air with the particles through a nozzle into an analysis chamber. In the analysis chamber particles with microorganisms are detected and the detected particles are ionized. The apparatus requires a relatively large number of particles, because many particles are lost in the transition from air pressure to the high vacuum used from mass spectroscopy. The efficiency of examined particles to captured particles may be as low as one percent.
From U.S. patent application 2005/0230615 a MALDI-IM (Ion Mobility) measurement technique is known wherein drops are formed with a Vibrating Orifice Aerosol Generator. Nothing is disclosed about micro-organisms in the drops. If the drops would contain micro-organisms, use of MALDI-IM would make it difficult if not impossible to identify these micro-organisms.
The use of Flow Field-Flow Fractionation (FFFF) in the preparation of a target plate for MALDI-MS is known from an article by Hookeun Lee et al, titled “Analysis of Whole Bacterial Cells by Flow-Field Flow Fractionation and Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectroscopy” and published in Anal. Chem. 2003, 75, 2746-2752. This document shows that FFFF can be used to separate intact bacterial cells to increase cell density for MALDI-MS, when a target plate is used.